Fix N+1 in issues API when loading parent epics

We preload the epic's group and route to prevent an N+1 when presenting
the issues' epics.

Changelog: performance
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61852

ee/app/models/ee/issue.rb

@@ -78,7 +78,7 @@ module EE
 
     class_methods do
       def with_api_entity_associations
-        super.preload(:epic)
+        super.preload(epic: { group: :route })
       end
 
       # override

ee/changelogs/unreleased/211373-fix-epics-n-plus-1.yml

+---
+title: Fix N+1 in issues API when loading parent epics
+merge_request: 61852
+author:
+type: performance

ee/lib/ee/api/entities/issue.rb

@@ -7,7 +7,7 @@ module EE
         extend ActiveSupport::Concern
 
         prepended do
-          with_options if: -> (issue, _) { issue.project.group&.feature_available?(:epics) } do
+          with_options if: -> (issue, _) { issue.project.namespace.group? && issue.project.namespace.feature_available?(:epics) } do
             expose :epic_iid do |issue|
               authorized_epic_for(issue)&.iid
             end

ee/spec/requests/api/issues_spec.rb

@@ -269,6 +269,28 @@ RSpec.describe API::Issues, :mailer do
         expect_response_contain_exactly(iteration_1_issue.id)
       end
     end
+
+    it 'avoids N+1 queries' do
+      stub_licensed_features(epics: true)
+
+      group.add_developer(user)
+
+      subgroup_1 = create(:group, parent: group)
+      subgroup_1_project = create(:project, group: subgroup_1)
+
+      create(:issue, project: subgroup_1_project, epic: create(:epic, group: subgroup_1))
+
+      get api("/groups/#{group.id}/issues", user)
+
+      control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) { get api("/groups/#{group.id}/issues", user) }
+
+      subgroup_2 = create(:group, parent: group)
+      subgroup_2_project = create(:project, group: subgroup_2)
+
+      create(:issue, project: subgroup_2_project, epic: create(:epic, group: subgroup_2))
+
+      expect { get api("/groups/#{group.id}/issues", user) }.not_to exceed_query_limit(control_count)
+    end
   end
 
   describe "GET /projects/:id/issues" do